Wind - Agroecology: The Ecology of Sustainable Food Systems - page 87

7 Wind

Wind is not always present as a factor of the environment,
but it is nevertheless capable of having very significant
impacts on agroecosystems. These impacts are a result of
the wind's ability to (1) exert a physical force on the plant
body, (2) transport particles and materials — such as salt,
pollen, soil, seeds, and fungal spores — into and out of
agroecosystems, and (3) mix the atmosphere immediately
surrounding the plants, thus changing its composition,
heat-dispersal properties, and effect on plant physiology.
When all these types of effects are taken into consid-
eration, what may seem a relatively simple environmental
factor becomes quite complex. Wind can simultaneously
have both positive and negative impacts, or be desirable
in some instances and undesirable in others. Wind is,
therefore, a challenging factor to manage.

The rotation of the earth alters the flow of these large-
scale circulation cells. Air currents are deflected to the
right of the pressure gradient, north of the equator, and to
the left in the south. This deflection is known as the
Coriolis effect . At the surface, the end result is winds that
tend to blow from the northeast and southwest in the
Northern Hemisphere, and from the southeast and north-
west in the Southern Hemisphere. These winds, typical of
certain latitudinal bands, are known as the prevailing
winds . They are shown in Figure 7.2.
Although they describe overall, macro patterns of
atmospheric circulation at the surface, the prevailing
winds are subject to a great deal of local and seasonal
modification. This modification is the result of a number
of factors, including the presence of mountain masses on
the continents and the temperature gradients created by
the differential heating and cooling rates of land and water.
All these factors together result in the formation of
large, mobile high-pressure and low-pressure air masses
that greatly influence the local wind patterns as they
move. In the Northern Hemisphere, air circulates around
high-pressure cells in a clockwise direction and around
low-pressure cells in a counter-clockwise direction. In
the Southern Hemisphere, the directions are reversed.
In both hemispheres, air flows outward from areas of
high pressure toward areas of low pressure.

ATMOSPHERIC MOVEMENT

The earth's atmosphere is constantly in motion, circulat-
ing in ever-changing, complex, and locally variable pat-
terns. This circulation is responsible for moving air
masses and driving the changes in weather. It is also
responsible for creating the surface air movement we
experience as wind.
The most basic process driving the atmosphere's
movement is the differential heating and cooling of the
earth's surface. In the equatorial regions, intense heating
of the surface and the atmosphere just above it causes the
air to expand and rise high into the atmosphere, creating
a zone of low pressure. Cooler surface air, further away
from the equator, moves in to take the place of the rising
air mass, while high in the atmosphere the heated air
moves poleward. In the polar regions, the opposite occurs.
Air at the colder poles cools much more rapidly higher in
the atmosphere, and descends to the surface, creating a
high-pressure zone and the movement of surface air
toward the equator.
As a result of the equatorial low-pressure zone and
the polar high-pressure zones, large cells of circulation
are created in each hemisphere, as shown in Figure 7.1.
The flow of air in the equatorial cells and the polar cells
creates an additional cell in the temperate region of each
hemisphere. As a result, there is a zone of low pressure
(rising air) at about 60°N latitude and 60°S latitude, and
a zone of high pressure (descending air) at about 30°N
and 30°S.

LOCAL WINDS

Winds are also generated by local conditions that have to
do with such factors as local topography and proximity to
bodies of water. In certain areas these winds are relatively
predictable.
In coastal areas in the summer, as well as around large
bodies of water such as lakes or reservoirs, daytime winds
(called sea or lake breezes) typically blow toward the land
because the nearby land mass heats up faster than the body
of water. The air above the land heats up, expands, and
rises, and then the cooler air over the ocean flows inland
to take the place of the rising air. At night the process can
reverse as the land mass cools more rapidly than the water,
and winds begin to move toward the water.
Slope winds are another form of local wind. In areas
of mountainous topography, as the land radiates heat back
to the atmosphere at night, the air close to the surface
cools as well. Since cooler air is heavier, it begins to flow

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